Such an imaging apparatus that can pick up even a subject having highly bright and dark mixed images has been developed. Such an imaging apparatus can output both of an image signal taking in a long exposure time (hereinafter referred to as a long-exposure image signal) and an image signal taking in a short exposure time (hereinafter referred to as short-exposure image signal) for each subject. A clear image for a dark region of the subject is obtained from the image signal having the long exposure time, while a clear image for a bright region of the subject is obtained from the image signal having the short exposure time.
A conventional imaging apparatus such as a television camera having the aforementioned image pickup element having a wide dynamic range combines the aforementioned long and short exposure time image signals at a predetermined ratio therebetween into a combined image signal in a television camera itself, and outputs the combined image signal therefrom. Not only a clear image for the dark region of the subject but also a clear image for the bright region thereof can be obtained from the combined image signal. Hereinafter, aforementioned imaging apparatus outputting the long and short exposure time image signals is referred to as a wide dynamic range television camera.
Such kind of wide dynamic range television camera is disclosed, for example, in JP-2001-094870A, JP-2001-094999A, JP-2000-350220A, JP-2001-094871A or JP2001-094872A.
In the related television camera having a wide dynamic range, since the camera itself includes a complicated image combination circuit, the size of the camera becomes large, the consumption power thereof is increased, and the cost thereof is increased. When this camera is used as a camera for an image monitoring system or a surveillance system, the above characteristics become disadvantageous.
Those Japanese documents disclose that a wide dynamic range imaging element is used for the wide dynamic range television camera. However, the long- and short-exposure image signals may also be obtained in each frame by using an ordinary imaging device such as a CCD (Charge-Coupled Device) without using the wide dynamic range imaging element. For example, the long-exposure image signal is produced by transferring charges stored in each pixel of the ordinary imaging element in a normal exposure time (e.g. a time period for one vertical synchronizing period except for a blanking signal time), and while the short-exposure image signal is produced by such that the stored charge is wasted halfway in a normal exposure time and the charge stored in the rest of exposure time is transferred by using an electronic shutter facility of the imaging element. The effective exposure time is arbitrarily varied by the electronic shutter facility.
Further, the related television camera having a wide dynamic range combines the image signals having long and short exposure times at a predetermined fixed combination ratio therebetween into a combined signal and outputs the combined signal. However, a monitor device side for receiving such a combined signal cannot separate the image signals having the long and short exposure times. When the combined image signal is transmitted from the related television camera via a communication network to a plurality of image monitor devices, all of the monitor devices have to receive the same combined signal at the same combination ratio, which results in that combined images to be displayed on individual monitor displays of the monitor devices become the same. For this reason, the related television camera having a wide dynamic range cannot meet such demands of the individual image monitor devices that one of the monitor devices wants to display the dark region of the subject more clearly or another of the devices wants to display the bright region of the subject more clearly.